(a) Field of the Invention
The present invention relates to an artificial pigment, specifically to an artificial pigment with excellent dry pick resistance and to a method of preparing the same.
(b) Description of the Related Art
Conventional coated paper is manufactured by coating an inorganic pigment such as clay, calcium carbonate, aluminum hydroxide (Al(OH)3), titanium oxide (TiO2) on paper. In preparing coated paper, a natural binder such as casein or starch and an artificial binder such as styrene-butadiene latex, polyvinyl alcohol or acrylic latex are used as an adhesive. Also, many kinds of additives such as a dispersion agent, a thickener, or a water-resistance agent are used. However, the most important components in manufacturing coated paper are an inorganic pigment and a binder. In order to obtain balanced properties of coated paper, an appropriate inorganic pigment and a binder should be used.
The most commonly used inorganic pigments are clay and calcium carbonate. Clay having plate shape has merits of high paper gloss and printing gloss. Calcium carbonate has merits of fluidity, dry pick resistance, Ink set-off, brightness of paper, and opacity.
Besides the inorganic pigment, an artificial pigment can be used in order to obtain coated paper with high quality. Conventionally, a latex of styrene polymer or an acryl-styrene copolymer is used as an artificial pigment. The artificial pigment has very high glass transition temperature so that it can increase the paper gloss of coated paper, opacity and micro smoothness. However, since the artificial pigment has little dry pick resistance and high binder demand, if a sufficient amount of binder is not used, material properties such as dry pick resistance of coated paper and printing gloss can deteriorate.
Recently, in order to improve productivity and save drying energy, as high concentration of solid content of coating color proceeded, viscosity of a composition for paper coating increased. Because of this, fluidity of coated paper decreased and thus it brought a reduction of work-operation.
In addition, to increase the manufacturing speed of paper, there has been considerable research to improve productivity and to provide an increase in printed materials by increasing coating speed. When coating speed is increased, shear strength during coating also increases and thus high-shear fluidity becomes more important. A high-shear means a shear speed of more than one, one thousands of a second and a low-shear means a shear speed of less than one, one hundreds of a second. The low-shear viscosity affects the transfer of the coating color and the coating.
As mentioned above, there is a problem that low-shear/high-shear fluidity of the coating color is a previous requirement for a high concentration of coating color and an increase in coating speed. In order to solve the above problem, there are two solutions: one is replacing a natural soluble binder having a large thickening effect, such as starch or casein, with an artificial binder and the other is increasing the ratio of ground calcium carbonate with fine particles that has a lower viscosity in terms of a pigment. However, although calcium carbonate has many merits, it is unfavorable in terms of paper gloss, printing gloss and smoothness so that it is difficult to increase weight. Also fluidity problems can be solved by controlling particle size of a binder. As the particle size of a binder decreases, the high-shear viscosity is lowered. This principle is applicable to the artificial pigment. That is, fluidity can be controlled by changing the particle size of an artificial pigment used so that it is easier to cope with the high concentration and change of coating speed than that of using an inorganic pigment only.
In addition, the artificial pigment has a better water-retention property than an inorganic pigment. The water-retention property is a property of coating color to maintain moisture against exterior strength. When the water-retention property is low, the solid content of the coating color continuously increases during coating so that work-operation has problems and irregular binder distribution in the coating layer such as binder migration can occur. However, since an artificial pigment is conventionally more surface-hydrophilic than an inorganic pigment, the moisture-holding property is excellent and thus, the work-operation and the quality of coated paper can be improved.
Recently, due to an increase in printed materials, the tendency of using high-speed printing, particularly in offset printing is gradually increasing. Also, as a requirement of coating color, the importance of dry pick resistance and reduction of the binder content is continuously increasing. That is, the coated paper should resist strong mechanical strength against the surface of the coated paper of pigments in order not to incur picking from the coating layer and the falling off of pigment during printing. That is, the coated paper should have a clear printing appearance. Since the destruction of the paper surface is more severe as the printing speed is increased and content of binder is lowered, a pigment adhesive having excellent dry pick resistance is required in order to prevent the above problem.
In addition, the importance of ink set-off is increasing. Ink set-off is a property, which represents ink-drying speed after printing. In the case of multi-color printing, printing is usually done with four colors of blue, black, red, and yellow. As the ink set-off increases, the time period to print the next color can be short, thus it improves the productivity of the printing. If the ink is not well dried and goes to the next step, print mottle or a post-smear phenomenon can occur.
Gloss is an important factor to increase commercial value and quality of printing paper. Gloss is divided into paper gloss of coated paper and printing gloss after printing. As the value of both of these factors increases, the paper develops a good appearance. Conventionally, as the particle size of a binder used in the coating color is increased, the arrangement of the inorganic pigments is made easier, so that paper gloss increases. Also, as the glass transition temperature of the binder is increased, paper gloss increases. Another method increase paper gloss is to lower the content of the binder in the coating color. However, when the paper gloss is increased by only the above methods, the dry pick resistance is decreased.
Another method increase paper gloss is to use an artificial pigment. Although the conventional artificial pigment has a high glass transition temperature and can increase paper gloss, they have demerits of low dry pick resistance and printing gloss. In order to complement these demerits, there can be provided artificial pigment having dry pick resistance. But, such a pigment has a lower paper gloss than an artificial pigment having a high glass transition temperature, or it has a high paper gloss but insufficient dry pick resistance.
Meanwhile, in order to increase the printing gloss, the porosity should be lowered and thus, it is required to hold the solvent on the surface, until after printing, until stability is achieved. Thus small particle size of the binder, a low glass transition temperature and a high binder content of the coating color are desired. In this case, ink set-off should be lowered. Also, when an artificial pigment having high glass transition temperature is used, since porosity increases, printing gloss reduces.
In addition, wet-pick resistance is also an important printing property in offset printing. In the offset printing, damping water is used. If the wet-pick resistance decreases, pigments can be exfoliated by the application of a strong physical strength during printing.
Another printing property required in offset printing is wet ink receptivity. As described above, since damping water is used in the offset printing, if the coated paper does not effectively absorb water during printing, ink that does not have compatibility with water will not adhere to the coated paper well, which results in a low degree of printing. Generally, wet ink receptivity and wet-pick resistance are opposite properties so that it is difficult to increase both of them simultaneously.
With an increasing tendency of using high quality magazines and mail-order catalogues, there is an increasing demand for a low-weight coated paper. A low-weight coated paper has a lower coating amount and is thinner and lighter than middle or high-weight paper so that it needs to have excellent stiffness and high paper gloss and printing gloss in order to have a high quality. Thus, the coating color used in low-weight paper may have a different composition from that of general coated paper and the binder used also has a higher glass transition temperature than the usual binder. Particularly, when the amount of an artificial pigment increases, higher stiffness can be obtained. However, since dry pick resistance reduces significantly, an artificial pigment having dry pick resistance can be used in order to complete the demerit. However, since an artificial pigment having dry pick resistance has a low glass transition temperature, stiffness and paper gloss is somewhat reduced.
As above, it is very difficult to prepare a coating color that can provide a coated paper with excellent printing properties. Also, coating and printing circumstances are becoming more strict.
Considering the above problems of the prior art, the present invention provides an artificial pigment having excellent fluidity, paper gloss, stiffness, dry pick resistance, ink set-off and printing gloss.
In order to achieve the above object, the present invention provides an artificial pigment of inverted core-shell structure comprising;
a) a shell of butadiene polymer including:
i) a seed of butadiene polymer having a glass transition temperature of xe2x88x9210 to 50xc2x0 C.; and
ii) a seed covering of butadiene polymer having a glass transition temperature of xe2x88x9210 to 20xc2x0 C.; and
b) a core of styrene polymer having a glass transition temperature of 40 to 120xc2x0 C.
Also, the present invention provides a method of preparing artificial pigment comprising:
a) preparing shell of butadiene polymer; and
b) preparing core of styrene polymer on the inner side of said shell of a) step.
More specifically, the present invention provides a method of preparing artificial pigment comprising:
a) a step of preparing shell comprising
i) preparing seed latex by emulsion polymerizing seed composition comprising styrene, 1,3-butadiene, ethylenic unsaturated acid monomer, vinyl cyanide monomer, monomer copolymerizable with these, and a chain transfer agent; and
ii) covering outside of said seed latex with polymer of seed covering composition by adding seed covering composition comprising styrene, 1,3-butadiene, ethylenic unsaturated acid monomer and a chain transfer agent to said seed latex and emulsion polymerizing them; and
b) a step of preparing core inside of said shell by adding core composition comprising styrene and ethylenic unsaturated acid monomer to said shell and emulsion polymerizing them.
Also, the present invention provides a coated paper composition comprising the artificial pigment.
In addition, the present invention provides a paper coated with the paper coating composition.
Hereinafter, the present invention will be explained in detail.
As a result of extensive research in the need of a new coating color, the present inventors have found that the above object can be accomplished by distributing butadiene having dry pick resistance to the surface of an artificial pigment with a new method different from the prior art. That is, after preparing latex by using butadiene having low glass transition temperature, styrene having high glass transition temperature is controlled to react inside, not on the latex surface, so that both paper gloss and dry pick resistance of coated paper can be satisfied. Thus, the present invention was accomplished based on this.
The artificial pigment of the present invention has an inverted core-shell structure that has different morphology from the prior art and is a polymer prepared from reacting latex made from butadiene with styrene having high glass transition temperature. The artificial pigment of the present invention shows high paper gloss and printing gloss while having more excellent dry pick resistance than other known artificial pigment, and improves dry pick resistance while maintaining stiffness.
In addition, the artificial pigment of the present invention can prepare aqueous coating color by mixing with an inorganic pigment, a binder, a thickener, and the other additives and thus high quality coated paper can be made by coating the coating color on paper. The inorganic pigment is a pigment such as titanium oxide, calcium carbonate, or clay and a pigment extender and so on. The amount of the artificial pigment is preferably 3 to 20 parts by weight based on 100 parts by weight of inorganic pigments.
The artificial pigment of the present invention comprises a shell polymerization step and core polymerization step that forms the core inside of the shell, more specifically, a triple-structure comprising a seed-preparing step as a first polymerization; a shell-preparing step that forms a covering on the seed as a second polymerization; and a core-polymerizing step that forms a core inside of the shell as a third polymerization and each step is formed by emulsion polymerization.
The seed prepared through the first step is characterized in having a low gel content and appropriate hydrophilicity; the second polymerization is characterized in having a low glass transition temperature and gel content and high hydrophilicity as well as a long polymerization time; and the third polymerization is characterized in having a monomer composition that is higher in glass transition temperature and lower in hydrophilicity than the second polymerization and a short polymerization time. Also, the third polymerization is characterized in penetrating into the latex prepared from the second polymerization to perform reaction. Thus, the artificial pigment of the present invention is prepared by effectively controlling gel content, hydrophilicity and polymerization time in each step so that the artificial pigment has a new structure, that is, the second polymer forms outer shell of the artificial pigment and the third polymer forms inner shell.
The preparation processes of the artificial pigment of the present invention will be explained in detail.
The first step is an initial polymerization process of seed. The seed comprises 35 to 90 parts by weight of styrene, 10 to 55 parts by weight of 1,3-butadiene, 1 to 18 parts by weight of ethylenic unsaturated acid and 0.5 to 15 parts by weight of vinyl cyanide monomer.
Styrene gives the copolymer appropriate hardness and wet pick resistance. When styrene is less than 35 parts by weight, a sufficient hardness and wet pick resistance cannot be obtained. The other hand, when styrene is more than 90 parts by weight, dry pick resistance and film strength is lowered.
1,3-butadiene gives the copolymer flexibility. When 1,3-butadiene is less than 10 parts by weight, the polymer becomes too stiff and when it is more than 55 parts by weight, stiffness is lowered.
The ethylenic unsaturated acid monomer is appropriately used for improving dry pick resistance of polymer and stability of latex particles. The composition ratio of ethylenic unsaturated acid monomer is preferably 1.5 to 18 parts by weight. When the amount is less than 1 part by weight, the above effect cannot be obtained and when the amount is more than 18 parts by weight, a problem such as polymerization stability can occur. The ethylenic unsaturated acid monomer is preferably unsaturated carboxylic acid or unsaturated polycarboxylic acid alkyl ester having at least one carboxylic acid.
The unsaturated carboxylic acid is preferably at least one selected from the group consisting of methacylic acid, acrylic acid, itaconic acid, chrotonic acid, fumaric acid, and maleinic acid and the unsaturated polycarboxylic.acid alkyl ester is preferably at least one selected from the group consisting of monoethyl itaconate, monobutyl fumarate and monobutyl malate.
The vinyl cyanide monomer improves printing gloss and the amount of it is preferably 3 to 10 parts by weight. Also, since the vinyl cyanide monomer has high hydrophilicity with ethylenic unsaturated acid monomer, it is preferable to control hydrophilicity within preferable amount in each polymerization step. The vinyl cyanide monomer is preferably acrylonitrile or methacylonitrile.
The seed preparation step of the present invention comprises styrene, 1,3-butadiene, ethylenic unsaturated acid monomer and 1 to 25 parts by weight of a monomer copolymerizable with vinyl cyanide monomer and 0.1 to 1.0 parts by weight of a chain transfer agent.
The copolymerizable monomer is preferably at least one selected from the group consisting of unsaturated carboxylic acid alkyl ester such as methylacrylate, methylmethacrylate, ethylacrylate, ethylmethacrylate, butylacrylate, or butylmethacrylate; unsaturated carboxylic acid hydroxyalkyl ester such as xcex2-hydroxyethyl acrylate, xcex2-hydroxypropyl acrylate, or xcex2-hydroxyethyl methacrylate; unsaturated carboxylic acid amide such as acrylamide, methacrylamide, itaconamide, or maleic acid monoamide or derivatives thereof; and aromatic vinyl monomer such as xcex1-methylstyrene, vinyl toluene, or p-methyl styrene. The unsaturated carboxylic acid alkyl ester gives the copolymer an appropriate hardness and improves film formability. When the amount is more than 25 parts by weight, it adversely affects wet pick resistance so that it is preferable to have 3 to 15 parts by weight. In addition, the unsaturated carboxylic acid amide and derivatives thereof have an improving effect for chemical stability, mechanical stability and wet pick resistance of copolymer latex and the amount is preferably 1 to 10 parts by weight.
The chain transfer agent of the present invention is preferably selected from the group consisting of mercaptan such as n-dodecyl mercaptan or t-dodecyl mercaptan.
The seed preparation of the present invention is performed by emulsion polymerization under the addition of additives such as a conventional polymerization initiator, an emulsifier, or an electrolyte to the above seed composition.
The seed of the artificial pigment of the present invention has preferably less than 80% by weight of gel content, more preferably 20 to 60% by weight. When gel content of seed is more than 80% by weight, the second and third polymerization steps followed by the seed preparation step cannot control structure so that sufficient dry pick resistance cannot be obtained.
The second step of the artificial pigment preparing steps of the present invention is a covering step of shell on the seed latex prepared from the first step. The composition comprises 20 to 85 parts by weight of styrene, 23 to 75 parts by weight of 1,3-butadiene, 0.5 to 18 parts by weight of ethylenic unsaturated acid monomer, and 0.1 to 3.0 parts by weight of a chain transfer agent. Also, it can further comprise 1.0 to 20 parts by weight of vinyl cyanide monomer, and 0.1 to 25 parts by weight of monomers copolymerizable with other monomers.
The shell prepared from the second step of the present invention preferably comprises 25 to 65% by weight of gel content, more preferably 40 to 55% by weight. When the gel content is less than 25% by weight, gloss and polymerization stability can be lowered and when the gel content is more than 65% by weight, the monomer of the third step cannot effectively penetrate into polymer inside of the second step so that dry pick resistance reduces.
The third step of the artificial pigment preparing steps is performed by adding new monomers and polymerizing them when the conversion ratio of monomer to polymer in the second step is 55 to 85%. If the conversion ratio is less than 55%, monomers in the second step and monomers in the third step are mixed and their structures are not apparently separated, so that paper gloss and stiffness can be lowered. If the conversion ratio is more than 85%, effective inverted core-shell structure cannot be obtained so that dry pick resistance reduces.
The monomer composition of the third step comprises preferably 85 to 99.9 parts by weight of styrene, and 0.1 to 15 parts by weight of ethylenic unsaturated acid monomer. It can further comprise 0.1 to 25 parts by weight of 1,3-butadiene, 0.1 to 1.0 parts by weight of a chain transfer agent, 0.1 to 10 parts by weight of vinyl cyanide monomer, and 0.1 to 8 parts by weight of copolymerizable monomer in addition to the styrene and ethylenic unsaturated acid monomer.
Each monomer used in the second step and the third step is the same as the material used in the first step of seed preparation.
In addition, core after the third step has preferably 40 to 85% by weight of gel content, more preferably 55 to 78% by weight. When the gel content is less than 40% by weight, molecular weight is reduced so that dry pick resistance reduces. When the gel content is more than 85% by weight, effective control of structure is difficult and film-formability is reduced so that dry pick resistance can be lowered.
Also, it is very important for the artificial pigment of the present invention to control thickness and glass transition temperature in each step during process. The thickness in each step can be regulated by controlling seed content and monomer content. If it is not properly regulated, the artificial pigment with preferable structure cannot be obtained. In addition, if glass transition temperature in each process is properly regulated, printing properties, such as dry pick resistance, printing gloss, and ink set-off and other properties such as paper gloss and stiffness can be effectively controlled.
A proper average particle size of the first step after seed polymerization during process is preferably 50 to 90 nm, more preferably 55 to 80 nm, a proper average particle size after the second step is preferably 130 to 260 nm, more preferably 150 to 240 nm, and an average particle size of the artificial pigment after the third step is preferably 170 to 300 nm, more preferably 190 to 260 nm.
In addition, proper glass transition temperature of seed of the first step is preferably xe2x88x9210 to 50xc2x0 C., more preferably xe2x88x925 to 30xc2x0 C., glass transition temperature of the shell polymer prepared from the second step is preferably xe2x88x9210 to 20xc2x0 C., more preferably xe2x88x925 to 10xc2x0 C., and glass transition temperature of core polymer prepared from the third step is 40 to 120xc2x0 C., more preferably 60 to 120xc2x0 C.
Even though it is difficult to show the final glass transition temperature after completion of polymerization as a specific value, it is theoretically 20 to 60xc2x0 C., more preferably 30 to 50xc2x0 C. When the temperature is less than 20xc2x0 C., paper gloss and stiffness is lowered, and when the temperature is more than 60xc2x0 C., dry pick resistance reduces.
In addition, polymerization temperature and time in the second and third steps during preparation of the artificial pigment of the present invention have very important role to control the structure of the artificial pigment. The polymerization temperature and time should be preferably controlled together to maintain a balance of thermodynamic preference and dynamic speed. Particularly, the polymerization temperature and time of the third step should be controlled to have 50 to 85% of conversion ratio right after the third step in order to obtain inverted core-shell structure. Other reaction conditions, such as a polymerization initiator, an emulsifier, and an electrolyte are the same as the known art in an emulsion polymerization.
In the artificial pigment of the present invention, polymer of the third step is formed inside of latex, not surface of latex and polymer of the second step forms a shell. That is, polymer of the second step having good dry pick resistance exists on the surface of latex prepared from the first step and polymer of the third step having good stiffness exists inside of latex so that the artificial pigment having excellent dry pick resistance and paper gloss can be obtained.
Hereinafter, preferable examples and comparative examples are presented for the sake of understanding. These examples, however, are provided to facilitate the understanding and the present invention is not limited to the following examples.